tests: Fix garnet and memcheck tests to use X86

These tests previously used "build/NULL" but due to changes in the
"Ruby" and "garnet_synth_traffic.py" scripts, "NULL" fails as the script
exists "X86TimingSimple" with MESI_Two_Level.

This change fixes the tests by compiling and using the correct
compilation of gem5. It shouldn't affect the tests in any negative way.
As far as I'm aware it does not matter what ISA is used for these tests.

Change-Id: I8ae84b49f65968e97bef4904268de5a455f06f5c

tests/gem5/memory/test.py

@@ -70,7 +70,7 @@ gem5_verify_config(
     length=constants.long_tag,
 )
 
-null_tests = [
+x86_tests = [
     ("garnet_synth_traffic", None, ["--sim-cycles", "5000000"]),
     ("memcheck", None, ["--maxtick", "2000000000", "--prefetchers"]),
     (
@@ -122,7 +122,7 @@ null_tests = [
     ("ruby_direct_test", None, ["--requests", "50000"]),
 ]
 
-for test_name, basename_noext, args in null_tests:
+for test_name, basename_noext, args in x86_tests:
     if basename_noext == None:
         basename_noext = test_name
     gem5_verify_config(
@@ -133,7 +133,8 @@ for test_name, basename_noext, args in null_tests:
             config.base_dir, "configs", "example", basename_noext + ".py"
         ),
         config_args=args,
-        valid_isas=(constants.null_tag,),
+        valid_isas=(constants.x86_tag,),
+        protocol="MESI_Two_Level",
         valid_hosts=constants.supported_hosts,
         length=constants.long_tag,
     )